Just as the tree is the unit of a forest and the book the unit of a library, ______ the molecule is the unit of matter.

1. A.
   as
2. B.
   like
3. C.
   so
4. D.
   thus

Just as the tree is the unit of a forest and the book the unit of a library, ______ the molecule is the unit of matter.

A. as                         B. like              C. so                                   D. thus